Download BBA IInd SEMESTER EXAMINATION 2008-09

B.Tech III (Third) Semester Examination 2012-13
Course Code: EEE304
Paper ID: 0113320
DC Machines and Transformer
Time: 3 Hours
Max. Marks: 70
Note: Attempt six questions in all. Q. No. 1 is compulsory.
1.
a)
b)
c)
d)
e)
f)
g)
h)
2.
Answer any five of the following (limit your answer to 50
words).
(4x5=20)
Show that the reaction of coupling magnetic field on the
electrical or mechanical system is essential for the
electromechanical energy conversion process.
State and explain with suitable examples, singly excited and
doubly excited magnetic field systems. Also mention their
applications.
What are the advantages of analyzing energy conversion
devices by field energy concept?
Give the materials and functions of the following parts of a
DC machine:
i)
Field poles
ii)
Yoke
iii)
Commutator
vi)
Commutating poles
Explain in brief how efficiency is obtained from Swinburne’s
Test.
Explain what would happen, if the dc motor is directly
switched on to the supply without any starter.
What is the difference between 3 single phase transformer
bank and a 3-phase transformer unit?
Discuss the conditions necessary for the successful parallel
operation of 1-phase transformers. How can you check these
conditions?
Develop the relationship between energy and co-energy in a
magnetic field system. Also give a diagram of flow of energy
in an electro-mechanical energy conversion device via a
coupling field
(10)
3.
Distinguish between self-excited and separately excited D.C.
generators. How are self-excited D.C. generators classified?
Give their circuit diagrams with voltage equations.
(10)
4.
A short-shunt compound D.C. generator delivers 100 A to a
load at 250V. The generator has shunt field, series field and
armature resistance of 130 Ω, 0.1 Ω and 0.1Ω respectively.
Calculate the voltage generated in armature winding. Assume
1 V drop per brush.
(10)
5.
Draw and explain the speed-torque characteristics of a dc
shunt, series and compound motors. Which characteristic is
more suitable for traction purpose and why?
(10)
6.
Explain with necessary diagrams how two 3-phase
transformers can be used to convert a 3- phase supply to a 2phase one. If the load is balanced on one side, show that it
will be balanced on other side.
(10)
7.
Two transformers A and B are joined in parallel to the same
load. Determine the current delivered by each transformer,
given: open-circuit emf 6600 V for A and 6400 V for B.
Equivalent leakage impedance in terms of the secondary =
(0.3 + j 3) Ω for A and (0.2 + j 1) Ω for B. The load
impedance is (8 + j 6) Ω.
(10)
8.
Derive an expression for saving in conductor material in an
autotransformer over a two-winding transformer of equal
rating. State the advantages and disadvantages of
autotransformers over two-winding transformers. Also write
its applications.
(10)